1. Field of the Invention
This invention relates to a seal device in a displacement measuring instrument, and more particularly to improvements in a seal device in a displacement measuring instrument, suitable for use in a linear type displacement measuring instrument, in which the seal device comprises: a hollow case formed with an opening along a moving line of one member whose relative movement is detectable, which is fixed to the other member whose relative movement is detectable; a connecting rod for imparting a displacement of one member to a displacement detecting mechanism housed in the hollow case through the opening; and seal members disposed at opposite sides of the opening of the hollow case, for shielding a penetrating portion of the connecting rod.
2. Description of the Prior Art
In general, in a measuring instrument for measuring a length or the like of an article, when a movement value of articles relatively movable with each other, such as a movement value of a measuring element to a main body and a movement value of a slider to a column, there have been known displacement measuring instruments, in which a main scale is secured on the one hand and a detector including an index scale is secured on the other hand, and a relative displacement value is read by an optical method or an electromagnetic method for example.
The displacement measuring instruments of the type described, working with high accuracy, and hence, being fine in the construction, there have been a number of problems for maintaining a predetermined accuracy and enhancing the enlarged adaptability to the environment of use, to thereby improve the value of practical use. More specifically, for example, in an optical displacement measuring instrument, in which a main scale and an index scale, both of which have graduations of a micro meter order, are used, it has been necessary that the dust-proof and liquid-proof measures be perfectly taken because a gap between the both scales be maintained in the micro meter order and a comparatively weak light source is used to save the electric power, or the like.
To this end, as shown in FIGS. 1 and 2, there has been adopted a displacement measuring instrument provided therein with a seal device comprising: a hollow case 20 fixed to one member whose relative movement is detectable, e.g., a bed 10 of a machin tool and formed with an opening 20a along a moving line of the other member, e.g., a workpice or a tool 12; a connecting rod 24 for imparting a displacement of the tool 12 to a displacement detecting mechanism 22 housed in the hollow case 20 through the opening 20a; and flexible lip members 26 and 28 made of rubber, the forward ends of which are adapted to come into abutting contact with the connecting rod 24.
In the drawing, designated at 30 is a main scale made of glass or the like, one end portion of which is inserted into a fixing groove 32 provided in the hollow case 20 in the longitudinal direction thereof and fixed thereto by use of a rubber rod 34, a bonding agent 36 and the like, and formed on a scale surface 30a with traverse fringe-shaped graduations, 38 is a slider movable in the longitudinal direction of the main scale 30 in a manner to hold a predetermined positional relationship with the main scale 30 by the virtue of sliding shoes 40 slidable on the surface of the main scale 30; 42 is an index scale fixed to a surface of the slider 38 opposed to the scale surface 30a of the main scale 30 and formed with traverse fringe-shaped gradnations similar to those of the main scale 30; 44 and 46 are a light emitting element and a light receiving element fixed to the slider 38 in a manner to interpose therebetween the main scale 30 and the index scale 42; and 48 is connecting means, in which a linear cantilever spring is used for example, for imparting a movement of the connecting rod 24 to the slider 38.
The above-described seal service, in which a pair of lip members 26 and 28 are used, has such a characteristic feature that this device can effectively prevent dust and oil drops from intruding thereinto, with a comparatively simple arrangement as compared with a device, in which the opening 20a of the hollow case 20 is sealed with magnetized metal belts or brushes. However, it has heretofore been contemplated to decrease the sliding resistance which adversely affects the measuring accuracy first of all in view of the penetrating chracteristics and the transverse moving characteristics of the connecting rod 24 for the relative movement, there have merely been provided the lip members 26 and 28 having the shapes identical with each other in a symmetrical arrangement. As the field of application of the displacement measuring instruments is enlarged in recent years, when the displacement measuring instrument is used as a feedback signal generator in a numerically controlled machine tool for example, there has been such a problem that machine oil intruding through a joined portion of the lip members 26 and 28 cannot be precluded.
With the conventional seal devices, there has also been such a problem that no removing mechanism works to remove the oil drops, which have intruded through a gap between the lip members 26, 28 and the connecting rod 24.